Wire forms the basic building block of the world's electric power system, including transformers, transmission and distribution systems, and motors. The discovery of revolutionary HTS compounds in 1986 led to the development of a radically new type of wire for the power industry; this discovery is the most fundamental advance in wire technology in more than a century. However, to date only short samples of the HTS-coated tape used in the manufacture of next-generation HTS wires have been fabricated at high performance levels. In order for HTS technology to become commercially viable for use in the power generation and distribution industry, it will be necessary to develop techniques for continuous, high-throughput production of HTS-coated tape.
A typical HTS tape is a 3-layer laminate of a support layer, a buffer layer and a HTS film. The support layer is typically composed of stainless steel or nickel and provides structural integrity and flexibility to the tape. The buffer layer buffer layer is disposed between the metal substrate and the HTS film to prevent reaction between the substrate and the HTS film and, importantly, acts as a template for epitaxial growth of the HTS film. Typical buffers are yttrium-stabilized zirconia (YSZ) or cerium oxide (CeO2). The HTS film is formed of, for example, yttrium-barium-copper-oxide (YBCO).
Presently, substrates are polished by well-known mechanical, chemical, or electrical means to achieve a high degree of planarity or surface smoothness. Planarity is important in the manufacture of integrated circuits and numerous processes have been developed to meet the requirement of achieving a high degree of surface smoothness in the silicon wafer substrates used in manufacturing such electronic components.
One type of polishing process, mechanical polishing, holds a stationary substrate on a rotating pad and presses it against a conformable rotating polishing pad. Mechanical polishing may be performed in conjunction with a chemically active abrasive solvent slurry, a process commonly referred to a chemical mechanical polishing, which provides a higher material removal rate.
The abrasive slurry is typically comprised of small very hard particles such as diamond or boron oxide. The size of the particles used and other parameters, such as rotation speed, duration and contact force determine the removal rate and eventual roughness of the substrate.
In order to achieve high current densities in the HTS film, one of the main requirements is that the substrate be very smooth, with minimal surface imperfections. When the coatings are in form of thin films (up to 10 microns), the surface quality of the substrate becomes critical. Defects in the surface of the metal substrate can lead to voids, delamination, undesired texture, and roughness in the coatings. Furthermore, these imperfections or roughness on the substrate surface can be transmitted through the intermediate buffer layer and lead to a defect in the HTS film, which in an HTS-coated tape application must have a high degree of planarity, with minimal surface imperfections over long lengths.
A number of surface polishing techniques are known to the art. One such polishing technique is described in Kubo, U.S. Pat. No. 5,938,502, dated Aug. 17, 1999 and entitled “Polishing Method Of Substrate And Polishing Device Therefor”. Kubo describes a method employing a polishing pad and a slurry. The polishing device includes a bed formed with a polishing pad on the surface and driven for rotation, a rotatable carrier for holding the substrate to be polished, and a slurry supply means for supplying a slurry as an abrasive to the surface of the polishing pad. The substrate is polished by the abrasive slurry and the polishing pad, while pressing the substrate held by the carrier onto the polishing pad. Kubo's polishing technique may require several passes to achieve the smoothness required for an HTS-coated tape application.
Kubo's technique however, is applicable only to stationary substrates and is not suitable for the polishing of long lengths of continuously translating substrate tape. Furthermore, in the manufacturing of HTS-coated tape it is preferable to complete the surface preparation of the substrate in one pass so as to increase throughput and reduce cost. Thus, another drawback of Kubo's polishing technique is that it requires several passes and is therefore not suitable to a process for polishing long lengths of continuously translating substrate tape.
Shendon, U.S. Pat. No. 6,336,851, dated Jan. 8, 2002 and entitled “Substrate Belt Polisher,” describes a flexible membrane-polishing belt against which a substrate for a semiconductor wafer is polished using chemical mechanical polishing principles. A fluidized layer is provided on a surface of a polishing member backing assembly, which urges the moving polishing membrane toward the substrate held in a polishing head. The linear motion of the belt provides uniform polishing across the full width of the belt and provides the opportunity for a chemical mechanical polishing to take place. Several configurations are disclosed. They include belts which are wider than the substrate being polished, belts which cross the substrate being polished, but which provide relative motion between the substrate and the polishing belt, and polishing belt carriers having localized polishing areas which are smaller than the total area of the substrate to be polished. Only a small area on the surface of the substrate is in contact with polishing membrane but the motion of the carrier with respect to the substrate is programmed to provide uniform polishing of the full substrate surface, as is each configuration described.
Jackson et al., U.S. Pat. No. 6,241,591, dated Jun. 5, 2001, and entitled “Apparatus And Method For Polishing A Substrate,” describes a polishing apparatus. Uniform pressure distribution allows a semiconductor substrate polished with the polishing apparatus to have reduced edge exclusion, and thus increased die yield.
Nagahara et al., U.S. Pat. No. 6,179,690, dated Jan. 30, 2001, entitled “Substrate Polishing Apparatus,” describes a chemical mechanical polishing apparatus that includes a rotating plate on which a substrate is received, and a polishing pad, which moves across the substrate as it rotates on the plate to polish the substrate. The load of the pad against the substrate, and the rotary speed of the plate, may be varied to control the rate of material removed by the pad.
Sarfaty et al., U.S. Pat. No. 5,741,171, dated Apr. 21, 1998 and entitled “Precision Polishing System,” describes a polishing system able to polish samples to accuracy within the sub micron range. The polishing system has applications in the semiconductor field for use in polishing silicon wafers during testing and quality control inspections.
However, none of these prior art polishing systems are capable of solving the problem existing during the manufacture of HTS tape, viz. how to polish a continuously moving length of material in a single pass.
It is an object of this invention to provide a polishing system amendable to continuous production of HTS tape in a one-pass operation.
It is therefore an object of the invention to provide a system and method for surface preparation of long lengths of metal substrates in a continuous manner.
It is another object of the invention to provide a polishing system and method for producing a surface roughness of the substrate that is of very high quality, suitable to achieve high current densities in long lengths of superconducting tapes, in a single polishing pass.